Sorry for the late Answer,
Finally i "solved" the Problem. Not in a perfect way but i van use it. You have to use the SP Flash Tool with the BROM Bypass Script wich grants you the fully Functionallity without having an "Authorized Account". If you get an Error while formatting you have to run the Script again.
With the SP Flash Tool you have to Format the "FRP-Partition", the "MI-Account Partition" and optionally the "User-Data-Partition". You can find the "Start Adress" and "Format Lengh" in the Scatter.txt file you find in the original ROM. You should deactivate your antivirus programm otherwise all operations will take pretty much more time to finish.
Do it offline, keep the computer and the phone disconnected from the internet, otherwise it could be relocked again. Xiaomi Servers stores a Blacklist of MI-Accounts are locked with the serial number (NOT the IMEI) of the phone so you have to provide that your phone connect Xiaomi over the internet.
I have done so with my MI-Account locked RN8P and now i can use it without any Problems and if you believe or not, the power consumation of the phone was increased, i can use it now for 3 or 4 Days without charging it. I deactivated all installed chinese Certificads in the Sercurity Options.
@gabit7 Did you find another Solution or wich issue you found?